The purpose of this project is the determination of the peptostreptococcal population in dental plaque from humans and experimental animals at different sites in the dentition. Antisera have been raised in rabbits against a stock ATCC strain of Peptostreptococcus anaerobius (27337) and several oral isolates capable of the reduction of proline to S-NH2 valeric acid. Following conjugation with fluorescein the antisera were used to examine sonicates of plaque removed from single sites in the dentition. The data so far suggest that these organisms can comprise a significant fraction (15% of the total cellular material) in certain sites and may thus exert a significant effect on the overall pH balance of plaque in these areas.